Liner for shallow trench isolation

ABSTRACT

A method of depositing dielectric material into sub-micron spaces and resultant structures is provided. After a trench is etched in the surface of a wafer, a silicon nitride barrier is deposited into the trench. The silicon nitride layer has a high nitrogen content near the trench walls to protect the walls. The silicon nitride layer further from the trench walls has a low nitrogen content and a high silicon content, to allow improved adhesion. The trench is then filled with a spin-on precursor. A densification or reaction process is then applied to convert the spin-on material into an insulator. The resulting trench has a well-adhered insulator which helps the insulating properties of the trench.

FIELD OF THE INVENTION

This invention relates generally to the field of integrated circuitfabrication, and more specifically to trench isolation and methodstherefor.

BACKGROUND OF THE INVENTION

Integrated circuit (IC) manufacturers increasingly face difficultieswith scaling and insulation between components with ever decreasingfeature sizes. Even though packing transistors closer is important tothe concept of increasing IC speed and decreasing size, they must stillbe electrically separated from each other. One method of keepingtransistors separate from each other is known as trench isolation.Trench isolation is the practice of creating trenches in the substratein order to separate electrical components on the chip. The trenches aretypically filled with an insulator that will prevent cross-talk betweentransistors.

Shallow trench isolation (STI), which is becoming quite prevalent inmodern IC design, uses trenches that are substantially narrower thanprevious isolation technology, such as LOCal Oxidation of Silicon(LOCOS). The size can vary, but a trench less than one half of a micronwide has become quite common. STI also offers smaller channel widthencroachment and better planarity than technologies used in earlier ICgenerations.

During the deposition process and subsequent steps, however, the trenchwalls can be damaged. A silicon nitride liner in an STI trench hassubstantial stress-relieving capabilities for the sidewalls of thetrench. Such liners are often used for high density ICs, such as dynamicrandom access memory (DRAM) chips, to protect the bulk silicon duringsubsequent process steps.

In order to provide good isolation properties, the trench is thentypically filled with an insulator such as a form of silicon oxide. Theoxide can be deposited in a number of methods, such as CVD, sputtering,or a spin-on deposition process. Spin-on insulators, or spin-ondielectrics (SOD), can be deposited evenly. Additionally, SOD materials,which often form silicon oxide after being reacted, carry less risk ofvoids in the resulting insulating material than other depositionprocesses. The SOD precursor is reacted to form silicon oxide using ahigh temperature oxidation process.

However, problems relating to the formation of the SOD are common andcan cause significant problems for IC designers. In particular,interface problems between the SOD and the trench walls are common.Accordingly, better methods of SOD integration are needed for trenches.

SUMMARY OF THE INVENTION

In one aspect of the invention, an isolation structure for an integratedcircuit is disclosed. The isolation structure comprises a trench formedwithin a substrate. The trench has sidewalls and a base. A siliconnitride layer is within the trench. A portion of the silicon nitridelayer at the sidewalls and base of the trench has a higher level ofnitrogen than a portion of the silicon nitride layer most removed fromthe sidewalls and the base of the trench. An insulating material iswithin the silicon nitride layer.

In another aspect of the invention, a method of forming an isolationstructure is disclosed. The method comprises forming a recess with abase and sidewalls within a substrate. A first nitride layer isdeposited within the recess and a depositing a silicon-rich secondnitride layer is deposited over the first nitride layer. A insulatorprecursor within the second nitride layer is deposited.

In another aspect of the invention, a method of isolating electricalcomponents in an integrated circuit is disclosed. The method comprisesforming a trench within a substrate. A silicon nitride layer isdeposited within and lining the trench. The silicon nitride layer isdeposited by adjusting the deposition of the silicon nitride layer toproduce a lower nitrogen content in an interior portion of the nitridelayer compared to an outside portion of the silicon nitride layer. Asilicon oxide precursor is deposited within the silicon nitride layer.

In another aspect of the invention, a method of isolating electricalcomponents on a substrate is disclosed. The method comprises forming arecess in the substrate and depositing a graded silicon nitride layerwithin the recess. Depositing the silicon nitride layer comprisesdecreasing a nitrogen percentage while depositing the silicon nitridelayer. An insulation precursor is deposited after depositing the siliconnitride layer. An insulator is formed from the insulation precursor.

A method of forming an isolation trench in an integrated circuit isdisclosed in another aspect of the invention. The method comprisesforming a trench in a substrate. The trench is lined with a siliconnitride layer, wherein the silicon nitride layer has a nitrogen contentby atomic weight of less than 34%. The method further comprisesdepositing an insulator in the trench after lining the trench

A method of forming an isolation trench is disclosed in another aspectof the invention. The method comprises lining a trench with a barrierlayer and depositing an adhesion layer over the barrier layer. Theadhesion layer has a silicon content by weight of between about 66% and88%. The method further comprises depositing a spin-on insulationprecursor in the recess after depositing the adhesion layer. Aninsulator is formed from the insulation precursor.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be better understood fromthe Detailed Description of the Preferred Embodiments and from theappended drawings, which are meant to illustrate and not to limit theinvention, and wherein:

FIG. 1 is a schematic, cross-sectional side view of a substrate with athin “pad oxide” grown over the surface of the substrate, a thickerlayer of silicon nitride (Si₃N₄), and a photoresist mask in accordancewith a starting point for preferred embodiments of the presentinvention.

FIG. 2 is a schematic, cross-sectional side view of the substrate ofFIG. 1 after a trench has been formed.

FIG. 3A is a schematic, cross-sectional side view of the substrate ofFIG. 2 with a silicon nitride bilayer within the recess according to anembodiment of the present invention.

FIG. 3B is a schematic, cross-sectional side view of the substrate ofFIG. 2 with a graded silicon nitride layer within the recess accordingto another embodiment of the present invention.

FIG. 3C is a close-up view of schematic, cross-sectional side view ofthe substrate of FIG. 3B with a graded silicon nitride layer.

FIG. 3D is a schematic, cross-sectional side view of the substrate ofFIG. 2 with a silicon nitride tri-layer within the recess according toan embodiment of the present invention.

FIG. 3E is a close-up view of schematic, cross-sectional side view ofthe substrate of FIG. 3D.

FIG. 4 is a schematic, cross-sectional side view of the substrate ofFIG. 3A with a layer of spin-on dielectric material filling the trench.

FIG. 5 is a schematic, cross-sectional side view of the substrate ofFIG. 4 after a curing and densification process.

FIG. 6 is a schematic, cross-sectional side view of the substrate ofFIG. 5 after etching back the oxide down to the top nitride surface.

FIG. 7 is a schematic, cross-sectional side view of a substrate with anisolation trench with a graded nitride layer according to a preferredembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Shallow trench isolation (STI) trenches in dynamic random access memorydevices are typically lined with a silicon nitride layer. However,silicon oxide formed from a spin-on dielectric (SOD) precursor has beenfound to adhere poorly to conventional barrier materials, such asstoichiometric silicon nitride, Si₃N₄. However, a nitrogen-poor materialwill not effectively protect the trench and the active area of the bulksilicon from damage from processing.

In part, the adhesion problem is due to poor re-bonding, which makes theinterface between the silicon oxide and the silicon nitride weak. Duringthe densification process, the SOD precursor bonds with the walls of thesurrounding material to form a blended interface which provides a stableisolation structure. During the densification, the weakest bonds, thosebetween silicon and hydrogen, are broken down first. When a silicon-richsurface is provided as an underlayer for the formation of a SOD layer,more silicon is available for re-bonding at the silicon nitride/siliconoxide interface.

In order to improve the adhesion, the preferred embodiments provide ahigher silicon content near the interior of the trench. Preferably, theportions of the liner closest to the trench walls and base comprise asilicon nitride with a relatively lower silicon content. The spin-ondielectric precursor is deposited after forming the liner and is reactedto form an insulator for the STI trench. A highly silicon-based adhesionlayer also helps alleviate insulator shrinkage problems that can arisefrom the densification process. An etch-back step, such as chemicalmechanical polishing (CMP), can then be used to smooth the trenchsurface. A general process flow for forming such an isolation trench isdescribed herein.

While silicon nitride and SOD materials are preferred, other materialsare also available. The skilled practitioner will appreciate that thepreferred materials have particular advantages, but the principles andadvantages of the methods described herein are applicable to othermaterials.

Forming a Trench

An introductory step is the creation of a trench, preferably for shallowtrench isolation (STI). The trench can be of varying widths, but in apreferred embodiment, the trench is less than a micron wide. As shown inFIG. 1, a semiconductor substrate 10, e.g., a silicon wafer, is providedand a thin “pad” oxide 12 is thermally grown on the substrate. In someembodiments, a thicker layer of a silicon nitride 14, preferably Si₃N₄,is formed over the pad oxide 12. The nitride 14 is preferably formed bychemical vapor deposition (CVD). This nitride layer 14 acts as a stopfor a subsequent etch back, such as a CMP process. Exemplary thicknessranges are between about 30 Å and 100 Å for the pad oxide 12 and betweenabout 200 Å and 1500 Å for the nitride layer 14.

As shown in FIG. 1, a photoresist mask 16 is applied the substrate 10 inorder to etch the trench. Photoresist is applied on the surface of thewafer. A reticle that blocks ultraviolet (UV) radiation is then placedover the wafer. The photoresist is then selectively exposed to UVradiation. Depending upon whether positive or negative resist isemployed, the developing solution washes away either exposed orunexposed regions. Using the photoresist mask, a recess, preferably atrench for isolation, is formed in the substrate. After the trench isetched, the photoresist mask 16 of FIG. 2 is removed by conventionalresist strip process. The trench depth is preferably between about 1,000Å and 10,000 Å, more preferably between about 2,500 Å and 6,000 Å. Thetrench can also be formed through the use of a hard mask or by othermethods. The skilled practitioner will appreciate that the trench can beformed by several different procedures.

The trench is preferably etched by an anisotropic etch, i.e. adirectional etch, which produces relatively straight, verticalsidewalls. An exemplary etch process is reactive ion etch (RIE). Asshown in FIG. 2, this method is quite accurate and straight. However,RIE can also damage the edges of transistor active areas, defined by thesidewalls 18 of the trench. In one embodiment (see FIG. 7 andaccompanying description), the sidewalls 18 and base 19 are oxidized,forming a thin oxide layer in order to repair any damage from theanisotropic etch process.

Applying the Silicon Nitride Layer

After preparing the trench, a silicon nitride layer is deposited to linethe trench. The nitride layer can be deposited by any of a variety ofdeposition methods, including CVD, atomic layer deposition (ALD), andsputtering. An additional method of deposition is ion-metal plasmadeposition, a sputtering process with magnetic collimation. Whilesilicon nitride deposited by CVD is described here, other materials anddeposition processes can be used to form a liner layer.

Stoichiometric silicon nitride (Si₃N₄) has a silicon content ofapproximately 60% by weight, and a nitrogen content of about 40% byweight. In a preferred embodiment, the silicon content closest to thetrench sidewall 18 is lower than the silicon content in the portion ofthe nitride layer farthest from the trench sidewalls 18. Conversely, inthis embodiment, the nitrogen content of the nitride layer close to thetrench sidewalls 18 is higher than the nitrogen content near theinterior of the trench.

In several preferred embodiments illustrated in FIGS. 3A-3E, the nitrideliners are formed by a CVD process. Skilled practitioners willappreciate that several precursors can be used to form silicon nitridelayers of varying compositions. Some nitrogen precursors include ammonia(NH₃), which is a widely used precursor for many deposition processes,and nitrogen (N₂), which is used in processes such as plasma CVD.Exemplary silicon precursors include dichlorosilane (DCS) and othersilane compounds. Preferably, DCS and NH₃ are used together to form asilicon nitride liner. The combination of these two precursors providesan excellent layer at relatively low temperatures.

As seen in FIGS. 3A-3E, the nitride layer can be deposited in severaldifferent methods. In a preferred embodiment, the nitride layer can bedeposited in a single wafer chamber. The proportion of the precursorscan be adjusted during the deposition of the nitride layer. In anexample employing CVD, the nitride layer deposition can begin with a DCSflow rate of approximately 90 sccm, and a NH₃ flow rate of approximately270 sccm. Using a single wafer chamber at between about 600° C. and 800°C., these flow rates will generally form a layer that is approximatelystoichiometric silicon nitride (Si₃N₄) and has approximately 60% siliconby atomic weight. In the course of deposition the nitrogen source flowis reduced, relative to the silicon flow, or the silicon flow isincreased relative to the nitrogen flow. For the given example, by thetime the deposition of the nitride layer is completed, the DCS flow rateis preferably approximately 180 sccm, and the ammonia flow rate isunchanged. A nitride layer formed with these settings will besubstantially more silicon-rich at approximately Si₃N₂, and a siliconcontent of approximately 80% by atomic weight.

The transition between these states can be accomplished in severalmethods. Distinct layers with increasing silicon content can be formedby stepped changes. In the illustrated embodiment in FIG. 3A, thenitride layer is formed as a bilayer 20 and 30 in two deposition steps.In the embodiment illustrated FIGS. 3B and 3C, the nitrogen content ofthe nitride layer 22 is gradually decreased. The resulting nitride layer22 has smoothly graded nitrogen and silicon percentages. In theillustrated embodiment of FIGS. 3D and 3E, a nitrogen-rich layer 21 isformed in the trench. A nitrogen-graded layer 25 is formed over the“nitrogen-rich” layer 21. A silicon-rich nitride layer 31 is formed overthe graded layer 25. Skilled practitioners will appreciate that thereare a myriad of methods and patterns for decreasing the relativenitrogen content of the nitride layer or increasing the relative siliconcontent.

The liner layer (20 in FIG. 3A) closest to the trench walls 18 and base19 preferably acts as a barrier layer. The nitrogen content by atomicweight close to the trench walls 18 is preferably between about 36% and52%, more preferably between about 39% and 48%. Preferably the siliconcontent by atomic weight close to the trench walls is preferably betweenabout 48% and 64%, more preferably between about 52% and 61%. The linerlayer (30 in FIG. 3A) furthest from the trench walls 18 preferably actsas an adhesion layer. The silicon content by atomic weight in thisadhesion layer is preferably between about 66% and 88%, more preferablybetween about 73% and 85%. The nitrogen content by atomic weight of theadhesion layer is preferably less than about 35%, more preferablybetween about 12% and 34%, most preferably between about 15% and 27%.However, the skilled practitioner will appreciate that the barrier layerand the adhesion layer need not be separate and distinct layers.

In a preferred embodiment shown in FIGS. 3A and 4-6, the liner isdeposited in the trench in two steps. First a “nitrogen-rich” layer 20is deposited, followed by a silicon-rich layer 30. In a preferredembodiment, the nitrogen-rich layer 20 is between about 30 Å and 200 Åthick, more preferably between about 50 Å and 100 Å. In a preferredembodiment, this nitrogen-rich 20 layer is deposited by CVD. For thenitrogen-rich layer 20, the gas flow for ammonia (NH₃) is preferablybetween about 200 sccm and 340 sccm. The gas flow rate fordichlorosilane (SiH₂Cl₂) is preferably between about 50 sccm and 135sccm. A preferred ratio of nitrogen precursor to silicon precursor isbetween about 2.5:1 and 3.5:1, more preferably between about 2.7:1 and3.3:1. Preferably, the temperature in a single wafer CVD chamber isbetween about 500° C. and 900° C., more preferably between about 600° C.and 800° C.

The “silicon-rich” layer 30 is preferably thinner than the nitrogen-richlayer 20 at between about 5 Å and 100 Å thick, more preferably betweenabout 10 Å and 20 Å. The ratio of nitrogen precursor to siliconprecursor for the silicon-rich layer 30 preferably decreases from theratio of the nitrogen-rich layer 20 by between about 40% and 60%, morepreferably by between about 45% and 55%. A preferred ratio of nitrogenprecursor to silicon precursor is between about 1.1:1 and 2:1, morepreferably between about 1.3:1 and 1.8:1. For the silicon-rich layer 30,the gas flow for ammonia is preferably between about 200 sccm and 340sccm. The gas flow rate for DCS is preferably between about 140 sccm and220 sccm. The temperature in a single wafer CVD chamber is preferablymaintained between about 500° C. and 900° C., more preferably betweenabout 600° C. and 800° C.

In an exemplary embodiment, a nitride layer is formed using twodeposition steps as described above. In the first step, a 60 Å thick“nitrogen-rich” layer 20 was formed during a CVD process. The depositionprocess took 60 minutes to form using gas flow rates of 270 sccm ofammonia and 90 sccm of DCS. After the first step, the gas flow rates arethen adjusted to 270 sccm of ammonia and 180 sccm of DCS. Thesilicon-rich layer 30 is formed in a process that takes 60 minutes toform a 60 Å thick layer. The temperature of the chamber was about 600°C. in this example.

In another embodiment illustrated in FIGS. 3B and 3C, the nitrogencontent in the nitride layer 22 is graded down from a high level nearthe walls 18 and floor 19 of the trench, gradually decreasing as thedeposition process continues. In this embodiment the deposition processis not stopped to change the gas flow rates; rather, the flow rates areadjusted during the deposition process. The nitrogen and silicon contentof the nitride layer 22 is discussed above. In one embodiment, the flowrates begin at rates similar to the rates of the “nitrogen-rich” layer20 described in reference to the embodiment of FIG. 3A, and end at ratessimilar to the rates for the “silicon-rich” layer 30 in that embodiment.In a preferred embodiment, the gradient will be linear across the entirenitride layer 22. Other gradient profiles can also be formed dependingupon the operation of the deposition, specifically the adjustment of theflow rates.

In an embodiment illustrated in FIGS. 3D and 3E, the nitride layerconsists of three layers. The layer closest to the trench walls 18 willbe a “nitrogen-rich” layer 21, as in previous embodiments. Preferablythe nitrogen content close to the trench walls 18 and floor 19 issimilar to previous embodiments. Preferably this layer is between about5 Å and 20 Å thick, more preferably between about 10 Å and 15 Å. Anintermediate layer 25 will be graded from nitrogen-rich to silicon-rich,similar to the embodiments described with reference to FIGS. 3B and 3C.The gradient profile can be linear or nonlinear depending upon the flowrates and other operation factors of the deposition equipment.Preferably the graded layer is between about 10 Å and 100 Å thick, morepreferably between about 20 Å and 50 Å. An outer layer will be asilicon-rich layer 31 with a thickness of between about 5 Å and 20 Å,more preferably between about 10 Å and 15 Å. Overall, the thickness ofthe trilayer is preferably similar to the total thickness of the nitridelayer of previous embodiments.

Trench Fill Process

Once a nitride layer of one of the above embodiments has been depositedwithin the trench, the trench can be filled with an insulator.Preferably the insulator is a non-conductive oxide, such as siliconoxide. In a preferred embodiment, the trench is filled with a spin onmaterial. Although the rest of the disclosure assumes use of theembodiment of FIG. 3A, the skilled artisan will appreciate that otherembodiments, such as those described with reference to FIGS. 3B-3E,could also be used.

In FIG. 4, an insulator precursor material 40 has been deposited withinthe trench. A spin-on deposition process is preferably used to depositthe precursor 40 into the remaining space in the trench, as shown inFIG. 4. The thickness of the precursor 40 will vary based upon the sizeof the trench, but in the illustrated embodiment the thickness of thematerial is preferably between 2500 Å and 5500 Å, more preferablybetween 3000 Å and 4500 Å.

Spin-on deposition uses liquid materials dripped on the substrate afterformation of the isolation trenches. The wafer is rapidly spun, whichspreads the liquid uniformly over the surface of the wafer after fillingthe low points on the wafer. An example of a spin-on material isSpinfil™ made by Clariant (Japan) K.K.-Life Science & ElectronicChemicals of Tokyo, Japan. This product is a polysilazane basedinorganic spin-on dielectric precursor. However, the skilledpractitioner will appreciate that many dielectric materials can be usedfor these purposes.

Once the insulating precursor 40 has been deposited into the trench, theprecursor 40 is converted to oxide. Clariant's Spinfil™ SOD precursor,based upon perhydrosilazane (SiH₂NH), has a recommendedconversion/densification recipe as follows:

1) 3 min of hot plate baking at 150° C.,

2) 30 min at 700-800° C. in steam ambient

3) Annealing for STI at 800-1000° C. in dry oxygen.

However, this process was found problematic for trenches that are verysmall, particularly where trenches of a variety of widths across thesubstrate are to be filled. With this process, during the subsequentetchings, CMP, and wet cleans, the trench-fill material has been foundto recess too much. SOD materials tend to have between about 10 percentand 30 percent volume shrinkage upon densification. This can be somewhatoffset by other layers in the trench feature expanding. A silicon-richlayer can expand during the SOD conversion process to offset volumeshrinkage as well as provide a re-bond improvement.

A more preferred densification process is described in an application bySmythe, et al. (filed Feb. 19, 2004, appl. Ser. No. 10/782,997), whichis hereby incorporated by reference. The densification process of thatapplication uses a ramped temperature process. A prepared wafer isplaced in a chamber. The wafer is preferably heated to an initialtemperature of between about 200° C. and 600° C., more preferablybetween 300° C. and 500° C. Preferably, steam is then turned on in thechamber. From the initial temperature, the heat ramps up to a targettemperature between approximately 800° C. and 1200° C., more preferablybetween 900° C. and 1100° C., and most preferably between 950° C. and1050° C. The increase of the temperature in the chamber is stopped whenit gets to this target temperature. The temperature can increaseapproximately between about 3° C. per minute to 25° C. per minute, morepreferably between about 8° C. and 20° C. During the escalation of thetemperature, the wafer is in an oxidizing environment, preferably anambient steam environment. After the temperature is ramped up, the waferis annealed for approximately 10 to 40 minutes, more preferably between15 min and 35 min, at the temperature plateau on steady state. In thepreferred embodiment, the wafer is annealed in a second oxidizingenvironment, preferably in a dry oxygen (O₂) environment. Finally, afterthe process is done, the wafer is removed from the chamber.

In this process the steam reacts with the polysilazane on the heatedsubstrate. As the temperature rises, the reaction begins to increase therate of oxidation. The chemical reaction associated with the densifyingprocess of the preferred spin-on dielectric, polysilazane, is shownbelow:Si_(x)N_(y)H_(z)+H₂O→SiO₂+H₂+NH₃

FIG. 5 shows the trench and surrounding area after the conversionprocess. Because the portion of the nitride layer 30 (FIG. 4) that is incontact with the insulating precursor 40 (FIG. 4) was silicon-rich, theinterface between the resulting insulator 44 and the liner layer 34 willbe smooth. The densification process consumes some of the silicon fromthe silicon-rich layer 30 (FIG. 4) and leaves a less silicon-rich layer34 and a layer of silicon oxynitride (SiON) 38 at the interface of thesilicon nitride layer 34 and the densified silicon oxide insulator 44.During the densification process, the gradient of the nitrogen contentand the silicon content may be smoothed out as a result of the highheat.

As seen in FIG. 6, the oxide 44 in the trench is preferably etched backwith a chemical mechanical polishing (CMP) process. The CMP processpreferably stops on the silicon nitride layer 34 when using a two layersilicon nitride layer. However, if the silicon nitride layer 30 isparticularly silicon-rich or was substantially oxidized by thedensification process, the CMP process can be stopped by the“nitrogen-rich” silicon nitride layer 20. When the layer has a gradednitrogen content from the sidewalls and base inward, the CMP process canalso stop within the silicon nitride layer.

Structure

An embodiment is seen in FIG. 6 after an etch back of the oxide. Throughthe use of the spin-on deposition process, the trench is preferablyfilled without voids, which can negatively affect the isolation effectsof the trench. It can be seen in FIG. 6 that the spin-on material 44 hasbeen reacted into silicon oxide, which provides excellent insulation.The silicon oxide 44 adheres well to the surface of the remainingsilicon nitride layer 34. Preferably, a silicon oxynitride layer 38 isformed at the interface of the silicon oxide 44 and the nitride layer34. In a preferred embodiment, the silicon oxynitride layer is betweenabout 5 Å and 20 Å thick, more preferably between about 10 Å and 15 Å.

Depending on the densification process, the nitrogen content gradientand silicon content gradient of the nitride layers 20 and 34 in thecompleted trench structure can be substantially altered. When the heatis high, such as in the exemplary process described above, the nitrogengradient may be somewhat smoothed by diffusion between the“nitrogen-rich” layer 20 and the remaining silicon-rich layer 34.However, even after processing, the silicon-rich layer 34 preferably hasa higher silicon content than the nitrogen-rich layer 20. Preferably,after processing, the nitrogen-rich layer 20 will have between about 48%and 64% silicon by atomic weight, more preferably between about 52% and61% silicon by atomic weight. The remaining silicon-rich layer 34 willhave preferably between about 50% and 85% silicon by atomic weight, morepreferably between about 60% and 80% silicon by atomic weight, and mostpreferably between about 70% and 75% silicon by atomic weight.

FIG. 7 is an illustration of a trench using a nitride liner 22 similarto that of FIG. 3B. The trench has been filled with an oxide 44, whichhas been etched back. In this embodiment, the graded nitride layer 36has provided an adhesion layer for formation of the oxide 44.Preferably, after processing, the portion of a resultant nitride layer36 closest to the trench walls 18 and base 19 will have between about48% and 64% silicon by atomic weight, more preferably between about 52%and 61% silicon by atomic weight. The portion of the nitride layer 36furthest from the trench walls 18 and base 19 will have preferablybetween about 50% and 85% silicon by atomic weight, more preferablybetween about 60% and 80% silicon by atomic weight, and most preferablybetween about 70% and 75% silicon by atomic weight.

Additionally, FIG. 7 illustrates an oxide layer 15 along the walls 18and base 19 of the trench resulting from oxidizing the surfaces of thetrench before depositing a graded nitride liner 22 similar to that ofFIG. 3B. The oxide layer 15 preferably has a thickness of between about30 Å and 100 Å. The oxidation of the trench walls 18 and base 19 servesto repair damage from the trench formation. The oxide layer 15 alsoprotects the active areas in the substrate 10 from damage that might becaused during processing. A similar oxide layer can be formed in otherembodiments described herein.

It will be appreciated by those skilled in the art that variousomissions, additions and modifications may be made to the methods andstructures described above without departing from the scope of theinvention. All such modifications and changes are intended to fallwithin the scope of the invention, as defined by the appended claims.

1. An isolation structure for an integrated circuit comprising: a trenchformed within a substrate, wherein the trench has having a plurality ofsidewalls and a base; a silicon nitride layer within the trench, whereina portion of the silicon nitride layer at the sidewalls and base of thetrench has a higher level of nitrogen than a portion of the siliconnitride layer most removed from the sidewalls and the base of thetrench; and an insulating material within the silicon nitride layer. 2.The structure of claim 1, further comprising an oxide layer between thesidewalls of the trench and the silicon nitride liner.
 3. The structureof claim 1, wherein the silicon nitride layer has a nitrogen contentthat is graded down from the sidewalls of the trench toward an interiorportion of the trench.
 4. The structure of claim 3, wherein the siliconnitride layer has a linear nitrogen content gradient from the sidewallsand the base of the trench toward the interior portion of the trench. 5.The structure of claim 1, wherein the silicon nitride layer comprises aplurality of layers, wherein each layer has a lower nitrogen contentthan a neighboring layer closer to the trench sidewalls.
 6. Thestructure of claim 1, wherein the insulating material is anon-conductive oxide material.
 7. The structure of claim 6, wherein theinsulating material is a silicon oxide material.
 8. The structure ofclaim 7, wherein the insulating material is a spin-on dielectric.
 9. Thestructure of claim 7, further comprising a silicon oxynitride layerbetween the silicon nitride layer and the silicon oxide material. 10.The structure of claim 9, wherein the silicon oxynitride layer isbetween about 5 Å and 20 Å thick.
 11. The structure of claim 1, whereinthe portion of the silicon nitride layer at the sidewalls and base ofthe trench has a silicon content by weight of between about 48% and 64%.12. The structure of claim 1, wherein the portion of the silicon nitridelayer at the sidewalls and base of the trench has a silicon content byweight of between about 52% and 61%.
 13. The structure of claim 1,wherein the portion of the silicon nitride layer most removed from thesidewalls and the base of the trench has a silicon content by weight ofbetween about 60% and 80%.
 14. A method of forming an isolationstructure comprising: forming a recess within a substrate, wherein therecess has a base and sidewalls; depositing a first nitride layer withinthe recess; depositing a silicon-rich second nitride layer over thefirst nitride layer; and depositing an insulator precursor within thesecond nitride layer.
 15. The method of claim 14, wherein forming therecess comprises forming a shallow trench isolation (STI) trench. 16.The method of claim 14, wherein forming the recess comprises using areactive ion etch process.
 17. The method of claim 14, furthercomprising densifying the precursor to form silicon oxide.
 18. Themethod of claim 14, further comprising oxidizing the base and sidewallsof the recess before depositing the first nitride layer.
 19. The methodof claim 14, wherein depositing an insulator precursor comprisesdepositing a spin on glass precursor.
 20. The method of claim 14,wherein forming a recess comprises using a reactive ion etch process.21. The method of claim 14, wherein depositing the first nitride layerand depositing the second nitride layer comprise using chemical vapordeposition (CVD) processes.
 22. The method of claim 21, whereindepositing the first nitride layer comprises flowing between about 200sccm and 340 sccm of ammonia into a single wafer chamber.
 23. The methodof claim 21, wherein depositing the first nitride layer comprisesflowing between about 50 sccm and 135 sccm of dichlorosilane into asingle wafer chamber.
 24. The method of claim 21, wherein depositing thesecond nitride layer comprises flowing between about 200 sccm and 340sccm of ammonia into a single wafer chamber.
 25. The method of claim 21,wherein depositing the second nitride layer comprises flowing betweenabout 140 sccm and 220 sccm of dichlorosilane into a single waferchamber.
 26. The method of claim 14, wherein depositing the firstnitride layer comprises depositing a nitride layer with a nitrogencontent by weight of between about 39% and 52%.
 27. The method of claim26, wherein depositing the first nitride layer comprises depositingsubstantially stoichiometric silicon nitride (Si₃N₄).
 28. The method ofclaim 14, wherein depositing the silicon-rich second nitride layercomprises depositing a nitride layer with a nitrogen content by weightof between about 12% and 34%.
 29. A method of isolating electricalcomponents in an integrated circuit, comprising: forming a trench withina substrate; depositing a silicon nitride layer within and lining thetrench, wherein depositing the silicon nitride layer comprises adjustingthe deposition of the silicon nitride layer to produce a lower nitrogencontent in an interior portion of the nitride layer compared to anoutside portion of the silicon nitride layer; and depositing a spin-ondielectric precursor within the silicon nitride layer.
 30. The method ofclaim 29, further comprising densifying the precursor to form adielectric.
 31. The method of claim 29, wherein depositing the siliconnitride layer comprises using a sputtering deposition process.
 32. Themethod of claim 29, wherein depositing the silicon nitride layercomprises using a chemical vapor deposition process.
 33. The method ofclaim 32, wherein depositing the silicon nitride layer comprisesdepositing a silicon nitride layer with a silicon gradient from a lowsilicon percentage at an early deposition stage to a higher siliconpercentage at a later deposition stage.
 34. The method of claim 32,wherein depositing the silicon nitride layer comprises forming aplurality of nitride layers.
 35. The method of claim 34, wherein formingthe plurality of nitride layers comprises adjusting a set of precursorgas flow rates step-wise.
 36. The method of claim 34, wherein formingthe plurality of nitride layers comprises pausing between depositingeach nitride layer.
 37. The method of claim 34, wherein forming theplurality of nitride layer comprises forming a first nitride layer andforming a second nitride layer with a higher silicon content than thefirst nitride layer.
 38. The method of claim 34, wherein forming theplurality of nitride layer comprises forming a first nitride layer witha low silicon percentage; forming a second nitride layer with a silicongradient from the low silicon percentage to a high silicon percentage;and forming a third nitride layer, wherein the third nitride layer has ahigher silicon percentage.
 39. A method of isolating electricalcomponents on a substrate, comprising: forming a recess in thesubstrate; depositing a graded silicon nitride layer within the recess,wherein depositing the graded silicon nitride layer comprises decreasinga nitrogen percentage while depositing the silicon nitride layer;depositing an insulation precursor in the recess after depositing thesilicon nitride layer; and forming an insulator from the insulationprecursor.
 40. The method of claim 39, wherein forming the recess in thesubstrate comprises forming an STI trench.
 41. The method of claim 39,wherein depositing the graded silicon nitride layer comprises using achemical vapor deposition process.
 42. The method of claim 39, whereindepositing the graded silicon nitride layer comprises depositing asilicon nitride layer with a nitrogen percentage by weight beginning atbetween about 39% and 52% and decreasing to between about 12% and 34%.43. The method of claim 39, wherein depositing an insulation precursorwithin the silicon nitride layer comprises depositing a spin-ondielectric precursor.
 44. The method of claim 43, wherein forming theinsulator from the insulation precursor comprises: curing the spin-ondielectric precursor in a curing chamber in a steam ambient environment,wherein the temperature in the curing chamber ramps from an initialtemperature of between about 200° C. and 600° C. to a target temperatureof between about 800° C. and 1200° C. at a rate of between about 3° C.and 25° C. per minute while the substrate is in the curing chamber; andannealing the substrate at a temperature of between about 800° C. to1200° C. for between 10 and 40 minutes after the substrate has beencured.
 45. A method of forming an isolation trench in an integratedcircuit, comprising: forming a trench in a substrate; lining the trenchwith a silicon nitride layer, wherein the silicon nitride layer has anitrogen content by atomic weight of less than about 34% at an innersurface; and depositing an insulator in the trench after lining thetrench.
 46. The method of claim 45, wherein depositing the insulatorcomprises depositing a spin-on dielectric over the silicon nitridelayer.
 47. The method of claim 46, wherein depositing the insulatorcomprises ramping a temperature setting from an initial temperature to ahigher final temperature.
 48. The method of claim 45, further comprisinglining the trench with a barrier layer before lining the trench with thesilicon nitride layer.
 49. The method of claim 48, wherein lining thetrench with a barrier layer comprises forming a silicon nitride layerwith a nitrogen content by atomic weight of between about 39% and 52%.50. A method of forming an isolation trench, comprising: lining a trenchwith a barrier layer; depositing an adhesion layer over the barrierlayer, wherein the adhesion layer has a silicon content by weight ofbetween about 66% and 88%; and depositing a spin-on insulation precursorin the recess after depositing the adhesion layer; and forming aninsulator from the insulation precursor.
 51. The method of claim 50,wherein depositing the adhesion layer comprises depositing a layer witha silicon content by weight of between about 73% and 85%.
 52. The methodof claim 50, wherein forming the insulator comprises forming siliconoxide in the trench.
 53. The method of claim 52, wherein forming siliconoxide comprises forming a spin-on dielectric.
 54. The method of claim52, further comprising etching back the silicon oxide.
 55. The method ofclaim 50, wherein lining the trench comprises forming a silicon nitridelayer.
 56. The method of claim 50, wherein lining the trench anddepositing the adhesion layer comprises using a chemical vapordeposition process.
 57. The method of claim 50, wherein depositing theinsulator precursor comprises depositing a spin-on precursor.
 58. Themethod of claim 57, wherein depositing the spin-on precursor comprisesdepositing a polysilazane based oxide precursor.
 59. The method of claim50, wherein forming the insulator from the precursor comprises ramping atemperature setting up from an initial temperature to a higher finaltemperature.